Along with the above answers, for the grand majority of cases, can = puedo and could = podría.

"Could" is the past tense of "can". So "He says I can go" becomes "He said I could go" in the past. It is also sort of a conditional or subjunctive form of "can" to show something is more tentative or hypothetical than "can" would imply, e.g., "I would go if I could", "Could you go, please?"

Usually you can use them pretty interchangeably, could is usually less definite than can. Example: I could make pizza. (it implies I could, but I might not or might not know how) where as: I can make pizza (this implies, I have the ability to do so and might do so.) when making a request of someone "could" is usually more polite. Example: Could you hand me that pen? is more polite than "can you hand me that pen" but you could use either.